1. Field of the Invention
The present invention is directed to the controlled production of microorganisms by photosynthesis in a closed photobioreactor containing a photosynthetic culture in a substantially sealed environment and wherein a reactant gas is recirculated through the algal culture.
2. Description of the Prior Art
Algae have been cultivated artificially for such diverse purposes as the production of food for animals and humans, the treatment of sewage and waste waters, and the accumulation of radioactive wastes. More recently, algal cultures have been used for the production of enzymes having industrial and research applications and for producing oils and other materials having nutritional value. Modern biotechnology offers an opportunity for the genetic modification of algae to yield cultures capable of producing a wide variety of useful materials.
Various methods and equipment have been employed for the artificial culturing of algae. Perhaps the simplest procedures have involved the use of shallow open ponds exposed to sunlight. Such ponds are subject to contamination by dust, other microorganisms, insects and environmental pollutants and provide minimal ability to control the degree of exposure to light, temperature, respiration and other important factors. A more sophisticated approach has involved growing algal cultures in plastic-covered trenches and ponds, optionally having electrically powered pumps and agitators. These configurations reduce the chances of contamination of the culture and permit more accurate control of temperature, respiration and other parameters.
Modern photobioreactor structures are constructed to optimize the photosynthetic process by providing a means for uniformly exposing the cells in the algal culture to the optimum amount of visible light. To accomplish this, prior photobioreactors have been built with sources of light, e.g., fluorescent tubes, optical rods etc., mounted in the photobioreactor, immersed in the algal culture. The light sources are positioned inside the photobioreactor taking into consideration such characteristics as the cell density and light path length.
The principal nutrient required for the algal culture in the photosynthesis process is inorganic carbon. In known photobioreactor systems, the algal cultures obtain their carbon from carbon dioxide, often bubbled through the culture medium. The carbon dioxide is often introduced in the medium through sparging tubes or other suitable means positioned near the bottom of the photobioreactors. The bubbling of the carbon dioxide often serves a dual function in that it aids in the circulation of the algal culture.
The presently known photobioreactors operate in what could be called an open-loop mode, that is, there is a free exchange of gases between the atmosphere and the interior of the photobioreactor. These photobioreactors are characterized in that they have open tops or tops which are not in sealed relation with the tank containing the algal culture. As the photosynthesis process occurs, the gases produced, oxygen being the main by-product of the biochemical transformation, are allowed to escape from the photobioreactor into the atmosphere.
Operating the photobioreactor in this open-loop mode is often satisfactory because the materials lost to the atmosphere, i.e., carbon dioxide, evaporated water comprising the liquid culture medium, are relatively inexpensive and are not environmentally harmful. In addition, oxygen produced in the photosynthesis reaction which if contained could result in overpressurization, is allowed to freely escape. However, when very expensive reactant gases such as carbon isotopes .sup.13 CO.sub.2 or .sup.14 CO.sub.2 are used in such systems, economically unacceptable losses result if these rare isotopes are allowed to freely escape. Furthermore, if deuterium oxide is utilized in the liquid culture medium, excessive evaporative losses of this expensive material may occur as well.